Globs of Goo are crafting ingredients that can also be thrown in order to slow down Creatures.
Green Globs of Goo can be collected as a loot and Pet-harvest from nearly all Creatures, and can also be crafted in the Crafting Menu from Mold after obtaining blocks of Mold from the Stalactite layer (requires a Stone Mining Cell or stronger Power Cell to be equipped).
Since update R59 on January 31st 2019, Globs of Goo of all colors can now be put on display too.
How to obtain
Common green Goo is randomly dropped by nearly all Creatures when they are killed (with the exception of all Keepa types, the Thing and several types of event Creatures that will either drop Halloween-themed or Christmas-themed items when being killed), but can as well be occasionally obtained by harvesting most Pets.
Tamed Ghost Creatures and event Creatures will provide you with the same harvest as their most common "brethren", but a lot more of it. Please note that Pets will always only provide you with their best pet-harvest if you have fed them their exact favorite type of Food as mentioned in their pet windows (point your cursor at them and either click the right mouse button or type "f" as the default key).
Since update R58 on December 19th 2018, green Goo can alternatively be obtained by crafting it in the Crafting Menu (to be opened by pressing "q" as the default key) from Mold that can be mined on the Stalactite layer. The recipe can be found in the category "Explosives".
Purple Goo and Orange Goo are special colored types of Goo that can only be crafted from Purple Blocks of Goo respectively from Orange Blocks of Goo since update R55 on August 22nd 2018, in the Crafting Menu since update R58 on December 19th 2018 in the "Explosives" category - these two colored variants of goo are both also simply called "Glob of Goo".
Different to their tooltip descriptions, neither Purple Goo nor Orange Goo can be obtained from any Creatures, instead they can solely be crafted from colored Blocks of Goo that have to be crafted at first in the Crafting Menu from green Goo, Stone Rods and Pigment.
Please note that Dark Lumite Stairs can only be crafted with purple Globs of Goo and the Purple Goo Cauldron too. Both of these crafts cannot be produced with common green Globs of Goo nor with orange colored Globs of Goo.
The Orange Goo Cauldron can only be crafted with orange Globs of Goo on the other hand, not with green or purple goo.
How to unlock the crafting recipes
The crafting recipe for green Globs of Goo will be unlocked when you obtain at least one block of Mold.
Mold is one of the blocks that the Stalactite layer underground is made of. It looks like Siltstone interveined with Moss. You'll find Mold in floors, walls and ceilings of caves on this layer, more of it can be obtained by creating mine shafts by digging.
The crafting recipe for Purple Globs of Goo, also simply called "Glob of Goo", can be unlocked in your Crafting Menu by obtaining at least one green Glob of Goo.
The same goes for Orange Globs of Goo that are also called "Glob of Goo" - these goo variants can be unlocked in your Crafting Menu by obtaining at least one green Glob of Goo as well.
How to craft
6 green Globs of Goo can be crafted from 1 Mold block each in your Crafting Menu (to be opened by pressing "q" as the default key).
6 blocks of Purple Goo can be crafted from Purple Blocks of Goo in the Crafting Menu (you can find the recipe in the "Explosives" category).
The crafting recipe for Purple Blocks of Goo is unlocked for free by crafting at least one green Block of Goo and obtaining (processing) Blue Pigment. Purple Blocks of Goo can then be crafted from green Globs of Goo, Blue Pigment and Stone Rods.
6 blocks of Orange Goo can be crafted from Orange Blocks of Goo in the Crafting Menu (you can find the recipe in the "Explosives" category).
The crafting recipe for Orange Blocks of Goo is unlocked for free by crafting at least one green Block of Goo and obtaining (processing) Red Pigment. Purple Blocks of Goo can then be crafted from green Globs of Goo, Red Pigment and Stone Rods.
Gauntlet Smash for "permanent" Globs of Goo
These goo globs (green, purple, orange) are special in that they are not sticky and won't slow down Creatures while still reducing fall damage.
Also, these Globs of Goo that are produced from Blocks of Goo per Gauntlet Smash do not vanish by themselves after a while, but will then stay where the Blocks of Goo had been placed (even in the air) and will also float if you pick up the block they're on.
These extraordinarily durable Globs of Goo can only be removed by using (basic) Excavators or (ordinary) TNT as long as these Explosives directly include the glob/s of goo in their area of effect. Excavators will even return half of the globs of goo they remove to you (which means that you might not get any, or only rarely any, if you only remove one glob of goo by using an Excavator).
On December 19th 2018, several block shape sets were added for green goo, orange goo and purple goo. All of these Shapes can be permanently transformed with a Gauntlet Smash.
Note: it has not been confirmed by the developers yet if this permanent transformation is a feature or a bug. It's possible that the transformation is supposed to be time-limited.
How to use
You can throw Globs of Goo, no matter if green, purple or orange, by putting them into your quickbar, select the according slot and then right-click while pointing your cursor at the desired destination.
Only one glob from a stack of goo will be thrown in a slightly curved trajectory wherever your cursor crosshairs points at, which will cause a sticky goo blot to appear on any solid block you've hit. However, if you hit any liquid with thrown Goo, even if only half a block in "depth", the Goo will vanish and have no effects.
A Goo blot will be of the same color of Goo glob that you have thrown, but it will always appear flat and horizontal, even when thrown at a wall or any vertical side of block or object. Goo blots cannot be picked up nor rotated. They will vanish by themselves after ca. 30 seconds.
Globs of Goo can directly be thrown at Creatures, for example at Keepas that can be hard to "catch", and will slow them down significantly for max. 5 seconds after you've hit them. Sticking to a Goo-blot will not stun or freeze Creatures and will also not prevent Creatures from attacking (or defending themselves) in close combat range though.
Goo-blots from thrown Globs of Goo are solid (you cannot pass through them) and whenever a Creature walks or runs over such a blot, the Creature will be slowed down just like it had been hit directly by a Glob of Goo.
Goo-blots are not sticky for player characters though, but bouncy. You can use them as spring boards until they will vanish.
Goo-blots can be thrown at Mushrooms or Tallgrass and the like without replacing them, they will also not take up the same place/block as torches and other such items do. Goo-blots do not burn, cannot freeze, nor can they be corrupted.
Common green Globs of Goo are also very common crafting ingredients. They are used in crafting Grand Arctek Chests (with 60 slots), Christmas-themed storage gift boxes, Arc Signs, all types of TNT (including Advanced TNT, Strong TNT and Super TNT), Flaming Skulls, Haunted Fireworks and other spooky stuff like Candy Skull Statues and Candy Skull Banners, Diamond Doors, Lumite Doors, lumite furniture, diamond furniture, and many building blocks like Asphalt, Tourmaline Glass, Decorative Stone Floors, Iron Roofs, Lumite Wall, Curl Iron Wall and others.
Since update R17, green Globs of Goo can be crafted into solid, but "elastic" and somehow bouncy Goo Blocks - green, orange and purple ones. These blocks can be used as jumping pads or to cushion the fall of player characters.
And, as mentioned above, since update R55 on August 22nd 2018, orange and purple Globs of Goo can be crafted too from their colored Goo Blocks and can then be thrown just like green common Globs of Goo.
Globs of Goo can be used as cushions to prevent falling damage just like Blocks of Goo can, as long as they will prevail.
Since update R59 on January 31st 2019, Globs of Goo of all colors can now be put on display on display containers like Placemats, Stone Wall Shelves, Hidden Temple Altars, Galactic Grav-Chambers, Frozen Containers, Holiday Decorative Trees, Snow Buried Containers, Holiday Mantles, Flower Pots, Wood Planters or the like.
Globs of Goo are not sticky when on display, cannot be used as cushions and will often be reduced in size. While they cannot be rotated while on display, most display containers can be rotated with all the items inside.
Warning: please do not throw Globs of Goo at any usable object like at doors or into doorways, at storage chests, crafting stations, teleporters or the like. There once was a known bug that was able to make Goo-blots prevail permanently in cases like these or could even make usable objects vanish. The bug has been fixed ever since, but there's no guarantee that any new patch won't be able to make this bug re-surface until the next hotfix.